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Muons influence supernova explosion mechanisms

Simulations throw new light on the role of the elementary particle

Super nova remnant Cassiopeia A with cooling neutron star

11.07.2018 |

Complex processes take place on the microphysical level when a star explodes and becomes a neutron star.According to the latest findings the role of muons in these supernovas must be reconsidered.The muons — elementary particles that were previously neglected — have an effect on the speed of the contraction to form the neutron star. Thus they also affect the neutrino-driven explosion mechanism. The findings were made by scientists from GSI and FAIR, TU Darmstadt, the Max Planck Institute for Astrophysics in Garching, and Indiana University in the USA.

What exactly happens when a star explodes and becomes a neutron star? Scientists use complex model calculations to study this question. Simulations of a neutrino-driven supernova explosion have now provided indications that the muon, an elementary particle that was previously unjustifiably neglected in the calculations, actually needs to be taken into account. This result has been published in the journal Physical Review Letters.

Muons have previously been neglected in simulations of supernova explosions because it had been assumed that they were not produced in significant quantities in such events. The scientists working with Prof. Gabriel Martínez Pinedo, a theoretical physicist at GSI/FAIR and TU Darmstadt, have shown in their publication that the temperature and the electrochemical potential, however, are such that muon production is possible. “That changes the composition of the particles in the stellar material and the neutrino emission,” says Martínez Pinedo. These two effects are based on the following mechanism: A neutron star forms in the interior of certain supernovae. As such a star forms, it attracts matter, and powerful gravitational forces are present. Electrons in the interior of the neutron star, however, work against the gravitation due to their mutual repulsion and so create pressure. A proportion of these electrons are now converted into muons. Because muons have a higher mass than electrons, they generate less counterpressure in the interior of the neutron star being formed. As a result, the contraction speeds up. The faster contraction gives rise to more heat, which results in more neutrinos being produced and emitted. That affects the explosion mechanism. “The models must therefore take account of muons, because they affect the supernova explosion mechanism,” concludes Martínez Pinedo.

Theoretical calculations often provide important reference points for laboratory experiments. This is also the case at GSI and FAIR. Cosmic matter can be created in the laboratory using the particle accelerators in Darmstadt. The HADES experiment and the future CBM experiment at FAIR can, for example, reach the temperatures and densities at which the muon production takes place during neutron star formation. Theoretical predictions could provide the experimental physicists with orientation when they evaluate their experiments. “For our next step we are planning simulations that will tell us more about the role of the pions,” says Martínez Pinedo, looking forward. “They could also be playing an important role that is not yet completely understood.”